Steam has almost never failed to launch a game for me. Once I think when it was decrypting something, L4D2 perhaps, it didn't launch the first time I clicked, the 2nd time it worked.
I have hundreds of games on steam and its my preferred platform. I almost never buy retail anymore, its more expensive and for what... a box? wow, that just takes up shelf space. Manuals are almost never included anymore, that went out years ago. The cd? Yay something more to lose or scratch up. And with the protection schemes nowadays, a cd means very little in terms of if you can play a game years from now when the company goes under. Heck the last few valve games had to decrypt the cd, just like they decrypted the download game (remember how some people who got stuff like hl2 ep1/2 early from a retail store couldn't play it since the auth server wasn't allowing it?).
That being said, steam DOES have some negatives. You can't sell games or give away your account per the TOS. Also per the TOS they can take away your games for any reason whatsoever. However, for the lower prices I usually pay for steam stuff (since I wait for weekend sales or package deals) I can live with not being able to sell stuff.